The present disclosure relates to a light emitting device, an illumination apparatus employing the light emitting device and a display apparatus also making use of the light emitting device. More particularly, the present technology relates to a light emitting device utilizing the electroluminescence of an organic material, an illumination apparatus employing the light emitting device and a display apparatus also making use of the light emitting device.
As a light emitting device capable of emitting light at high luminance when driven by a low DC (direct current) voltage, the light emitting device making use of the electroluminescence of an organic material draws much attention, and the research of such a light emitting device has been carried out extensively as well as intensively. In the following description, the light emitting device making use of the electroluminescence of an organic material is referred to as an organic EL device. The organic EL device has a structure in which an organic layer is sandwiched between a light reflection electrode and a light transmission electrode. The organic layer includes a light emitting layer generally having a thickness in a range of several tens of nm to several hundreds of nm. Light emitted by the light emitting layer of such an organic EL device experiences interferences in the device structure before being fetched by an external device. In the past, attempts have been made to improve the light emission efficiency of the organic EL device by utilizing such interferences.
There has been proposed a technology disclosed in Japanese Patent Laid-open No. 2002-289358. In accordance with this technology, interferences of light emanating from the light emitting layer in a direction toward the light transmission electrode and light emanating from the light emitting layer in a direction toward the light reflection electrode are used to improve the light emission efficiency by setting the distance from the light emission position to a light reflection layer at such a value that light having a light emission wavelength resonates. In this way, the light emission efficiency can be improved.
In accordance with a technology disclosed in Japanese Patent Laid-open No. 2000-243573, light reflection on a boundary face between a light transmission electrode and a substrate is also taken into consideration in prescribing two distances. The two distances are a distance from the light emission position to a light reflection electrode and a distance from the light emission position to the boundary face between the light transmission electrode and the substrate.
In accordance with a technology disclosed in PCT Patent Publication No. WO01/039554 pamphlet, interferences caused by multiple light reflections between the light transmission electrode and the light reflection electrode are used to improve the light emission efficiency by setting the thickness of a layer provided between the light transmission electrode and the light reflection electrode at such a value that light having a desired wavelength resonates.
In accordance with a technology disclosed in Japanese Patent No. 3508741, as a technique for improving the viewing-angle characteristic of the chromaticity point of the white color in a display apparatus constructed by combining light emitting devices each capable of raising the light emission efficiency by making use of a resonator structure, there has been proposed a method for controlling the balance of attenuations of three colors, that is, the R (red), G (green) and B (blue) colors, by adjusting the thickness of an organic layer.
In accordance with the technologies described above, however, in the organic EL device making use of interferences of emitted light in order to increase the light emission efficiency, if the bandwidth of an interference filter of fetched light h becomes narrow, the viewing-angle dependence of the light emission characteristic increases as evidenced by, among others, the fact that the wavelength of the light h is much shifted when seeing the light emitting surface from a point separated away in an inclined direction and the fact that the light emission strength deteriorates.
On the other hand, there has also been proposed a technology disclosed in Japanese Patent Laid-open No. 2006-244713. In accordance with this technology, interferences caused by the phase of light emitted by a light reflection layer of an organic light emitting device having a monochromatic spectrum with a narrow band and a single light reflection layer provided on the light emanation side are set so as to give a reverse phase for a center wavelength. In this way, color-phase changes caused by variations in viewing angle can be avoided. In this case, the number of light emission wavelengths of one device and the number of light reflection boundary faces are each limited to one so that it is possible to sustain the luminance of monochromatic light and the viewing-angle characteristic of the monochromatic light. However, there is no band which is broad enough for eliminating changes of the color phase. In addition, if an attempt is to be made to broaden this band, it is necessary to enhance the degree of mutual cancellation by raising the reflectance. In this case, however, the efficiency deteriorates considerably.